Why and How to use Partitions?

Whereas I'm fairly computer-literate, I'm a newbie when it comes to partitions, and have a question please

What is the purpose of partitions? I have a one-year-old desktop with 80GB hd, only one drive, no partitions and plenty of free space.

I have a newish laptop with 40GB hd, pre-partitioned into 2 drives both of 18.6GB each. On the laptop, C drive now has only 2.6GB free and drive D is fully free, so I'm getting a bit worried about space here.

In simple terms please, why do laptop makers have a tendency to partition drives? And why and how does one use partitions?

I'm on WinXP, 512 RAM.

 

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